1. Field of the Invention
This invention relates to an image forming apparatus capable of being used in a printer, a copying machine or a facsimile apparatus of an electrophotographic type.
2. Related Background Art
In Japanese Patent Publication No. S62-45552, Japanese Patent No. 2878322 and Japanese Patent No. 2978556, there is disclosed a developing apparatus having a plurality of developing sleeves provided in opposed relationship with an electrophotographic photosensitive member. When a plurality of developing sleeves are used, development by the respective developing sleeves becomes possible at a plurality of developing positions and therefore, the amount of toner which contributes to the development of an electrostatic latent image on the photosensitive member can be increased. When the amount of toner contributes to development is great, it becomes easy to increase the density of a toner image.
Such a developing apparatus can secure a sufficient developing property even if the developing sleeves are not rotated at a high speed and therefore, is suited for a high-speed image forming apparatus. As the plurality of developing sleeves, two developing sleeves are popular by the reason that the complication of the construction of the developing apparatus is avoided. The two developing sleeves are disposed on the upstream side and downstream side, respectively, along the direction of rotation of the photosensitive member.
On the other hand, in Japanese Patent Application Laid-Open No. 2000-172027, there is proposed a consumption mode in which a toner near a developing sleeve is discharged to an electrophotographic photosensitive member except during image formation (during non-image formation). As the purposes of executing the consumption mode, mention may be made of realizing a longer life of the developing apparatus within a long-period range, and improving a cleaning property by the supply of the toner to other portions, such as a transferring portion and a cleaner portion in the image forming apparatus, than the developing apparatus.
Now, in the developing apparatus having two developing sleeves opposed to the photosensitive member, the consumption rate of a developer per sheet of image individually differs depending on the developing condition of the developing sleeve on the upstream side and the developing condition of the developing sleeve on the downstream side. Also, the consumption rate of the developer per sheet of image, even when the developing conditions of the developing sleeves on the upstream side and the downstream side are the same, may differ depending on the coverage rate of an image outputted. In the case of the output of a solid image, however, the consumption rate of the developer becomes a substantially constant rate.
Generally, the consumption rate of the developer tends to be higher on the upstream side than on the downstream side. This is because although on the upstream side, the toner does not adhere to an electrostatic latent image carried to a developing position, the toner has already adhered to the electrostatic latent image when carried to a developing position on the downstream side on the upstream side and therefore, the intensity of an electric field for making the toner adhere is correspondingly low. Also, another reason is that the time required for the electrostatic latent image to arrive at the developing position is longer on the downstream side than on the upstream side and therefore, a minute reduction in the potential of the latent image with the lapse of time which is called the dark attenuation of the photosensitive member becomes great. If the toner is made to fly on the upstream side and a small amount of toner is added on the downstream side and at the same time, the action of putting the toner on the electrostatic latent image on the photosensitive member in order is given, the consumption rate on the upstream side tends to become greater than that on the downstream side, when viewed also from the characteristic that an image of a high quality is obtained.
However, when the consumption rate of the developer is greater by the developing sleeve on the upstream side than by the developing sleeve on the downstream side, the following problem arises.
When images are confirmed within a long-period range, there occurs the inconvenience that the output density is lowered or the fog phenomenon increases. The reason for this is that on the downstream side, the photosensitive member to which the toner has adhered is rotated from the upstream side and therefore, including the fact that part of the toner having adhered on the upstream side is stripped off, the toner consumption rate on the downstream side is lower than the toner consumption rate on the upstream side. Therefore, the toner consumption rate near the developing sleeve on the downstream side is small as compared with the upstream side, and only the appropriately charged toner becomes liable to be consumed, and the relatively inappropriate toner comes to be gradually accumulated near the developing sleeve on the downstream side. Here, the relatively inappropriate toner (faulty toner) means a toner of a composition ratio which is not in the initial toner, and for example, in the case of a one-component magnetic toner, it is the phenomenon that the ratio of an abrasive material or a charge controlling agent as an extraneous additive becomes great or small as compared with the initial toner, or the phenomenon that the mean particle diameter of the toner becomes great or bipolar as compared with the initial stage. Also, when the toner consumption amount decreases, the amount of toner stagnating near the developing sleeve becomes great, and this sometimes leads to a case where the toner receives the heat in the interior of the image forming apparatus or stress such as the frictional contact by the rotation of the developing sleeve, and becomes different in developing characteristic from the initial toner. As the overall result of these, there arises the problem in a long-period range that the output density is lowered or the fog phenomenon increases.